In Quake II, the Big Gun is a key part of Stroggos' planetary defense system. It can unleash a powerful ElectroMagnetic Pulse (E.M.P.) blast capable of disabling or destroying nearly any type of spaceship, including the small SMC Drop Pods used by the Human Marines. Unaware of its capabilities and/or believing it to be ineffective against their Drop Pods, the Marines launched an enormous assault against Stroggos.
The Big Gun, however, proved to be very active and highly effective against even the smallest targets, managing to disable almost every Marine Pod upon entry into the atmosphere, scattering squads of Marines far and wide, stranding survivors miles away from their squad-mates and objectives, enabling the Strogg ground forces to crush the assault almost before it even began. Those that survived the landing operation were either killed immediately, or were taken prisoner. The only identified survivors of the massacre were Bitterman, who went on to complete Operation Alien Overlord by himself; Joker, who would prove successful in his mission to locate and neutralize the Strogg's waiting counter-fleet; and Stepchild, who went on to foil the Strogg's plan to trap the Human fleet in orbit, by destroying the Gravity Well. Each of these marines acted on their own, having become separated from their ill-fated squads during atmospheric entry
The Big Gun was not the reason any of the player characters was separated from their platoons. Bitterman was blindsided by another marine whose pod smashed into his. Joker's pod hit an asteroid during atmospheric entry, which actually saved his life because the rest of the squad was wiped out by Strogg ground forces. Stepchild chased a Strogg Gorgon fighter and fell through the hole opened by its crash landing.
The destruction of the Big Gun was one of the central objectives of Operation Alien Overlord, the disabling of which allowed the orbiting capital ships Phobos and Deimos to send fighter and bomber squadrons down to the planet below to perform strafing runs and airstikes on important targets (such as a fuel pod in a canyon outside Cerberon's main hangar complex, or a fuel production plant), damaging Strogg infrastructure and hampering their war effort.
Defense & Destruction
The Big Gun and the Capital City are defended by a deadly laser field that fries anything that touches it. The computer that controls the field is found in a vault below the Jail complex to the south of the Big Gun. Once the field is down, ground forces can attack it, but the Gun will destroy any ships that get past the atmosphere. Since the Big Gun is so important to the Strogg, the only locations that can have entrances to it are the Reactor and Hangar.
The player will gain access to the Big Gun after destroying the Main Reactor in the Power Plant by replacing its coolant with toxic waste. The main defense within the Big Gun comes in the form of a Hornet backed up by a few Flyers. Once the Hornet has been defeated, the player may then reach the main controls of the Laser Guard. Once activated, he/she has 10 seconds to escape the Big Gun via a tram system before it is destroyed. The explosion causes a chain reaction which annihilates the Big Gun and levels the surrounding area, including the Power Plant breaking down the entire of Strogg defense around the area except to the north.